1. Field of the Invention
The present disclosure relates to a magnetic survey system using an unmanned aerial vehicle, and more particularly, to a 3-dimensional airborne magnetic survey system and a 3-dimensional airborne magnetic survey method using the same.
2. Description of the Related Art
Typically, an airborne magnetic survey using a helicopter or a fixed-wing aircraft has merits in that a wide survey area may be surveyed in a rapid time.
However, since flight is required to be made high in a certain height or higher from a target object (the surface of the earth) in order to secure stability, it has many limitations to obtain data of high resolution. Accordingly, the airborne magnetic survey is mainly used for early wide area survey among many survey stages.
As a survey method with high stability such as an unmanned aerial vehicle has been recently developed, a method is now being developed which is capable of obtaining high resolution data by lowering the survey altitude near the earth in order to overcome the weakness of the airborne magnetic survey.
In such a way, a compact survey is performed within a narrow area with the low altitude airborne magnetic survey recently being developed in comparison to the existing method. This leads to researches trying to derive an interpretation result for a subsurface structure, which has higher resolution compared to the survey result obtained by using the existing fixed-wing aircraft.
Accordingly, it may be said to be an important research project to try to interpret structural magnetic susceptibility model having higher resolution by using magnetic survey data for interpreting a subsurface magnetic susceptibility structure.
Typically, the purpose of airborne magnetic survey is to construct a distribution map of magnetic anomaly over a wide survey area.
In other words, major concern is to construct a magnetic anomaly map. Accordingly, survey points for the airborne magnetic survey data have two-dimensional distribution. On the contrary, the newly proposed method is a method of acquiring magnetic survey data three-dimensionally rather than as 2 two-dimensional planar data.
In other words, for the typical survey methods, one magnetic value is obtained at an identical x, y (or latitude & longitude) coordinate, while the newly proposed method may acquire various survey data whose altitudes are different even at the identical x, y (or latitude & longitude) coordinate.
Such a 3-dimensional survey method can reduce the errors in calculation, which may occur when numerically calculating 3-dimensional data using 2-dimensional data or errors that may occur from anomaly bodies of residual magnetization, and include all merits that the 3-dimensional data has. As for the merits of the 3-dimensional data, data having various distances from magnetic anomaly bodies distributed underground are included in the same inversion processing and data used for the inversion processing have various resolutions, and therefore distribution efficiency of input data for the inversion processing becomes improved. Due to this, it is effective to reduce non-uniqueness of magnetic inversion interpretation performed to interpret a subsurface magnetic susceptibility structure, and it is effective to predict magnetization direction of anomaly bodies of residual magnetization, which is different from that of the geomagnetic field and is difficult to predict with the 2-dimensional data.
For example, in a case where it is assumed that anomaly bodies having various sizes and shapes illustrated in FIG. 1 are located at different areas in different depths, magnetic anomaly patterns appearing on the earth have similar sizes or patterns to each other like magnetic anomaly curves in the drawing and may not be distinguishable.
In other words, since sizes and shapes of a small anomaly body near the earth and a large deep anomaly body are identical, a number of errors (non-uniqueness of the inversion processing) may occur in determining one among various anomaly bodies in the inversion processing.
In detail, a magnetic survey data typically has a characteristic of having higher resolution, as a distance from a survey point to a target body is shorter.
This is because when two poles having magnetic pole strengths of m1 and m2 are distant from each other, a magnetic force F between the two poles is
      F    =                  1        μ            ⁢                                    m            1                    ⁢                      m            2                                    r          2                      ,where μ is permeability, according to Coulomb's law, and a magnetic field H used for magnetic survey is defined as a magnetic force applied to a unit pole and is a magnetic field
  H  =            F              m        2              =                  1        μ            ⁢                        m          1                          r          2                    at a point distant away from m1.
In other words, a magnetic field value actually surveyed in the magnetic survey is reversely proportional to square of distance away from a target body and becomes significantly reduced as farther away from the target body.
Considering such a principle, variation of an anomaly value according to the size of the anomaly body and a reduction effect according to a distance are merged to allow the non-uniqueness of the inversion processing to appear.